Hepatitis delta virus (HDV) is a unique human pathogen that serves as an important model system for both virology and RNA biology. The goal in this proposal is to understand the mechanisms by which HDV RNA and protein structures regulate the specific editing of HDV RNA by a host adenosine deaminase, ADAR1. Understanding these mechanisms will provide a basis for advancing our understanding of HDV replication and pathogenesis, as well as RNA editing via adenosine deamination, which is increasingly recognized as an important post-transcriptional regulatory mechanism for both cellular and viral genes. HDV uses editing to extend the reading frame of the viral protein to make a form, HDAg-L, which both forms virus particles for secretion and inhibits viral RNA replication. Thus, RNA editing at the viral amber/W site plays a central role in the HDV replication cycle. The RNA sequences and structures required for editing, and the sequences added to HDAg-L that are essential for its packaging and inhibitory functions, are distinguishing features of the three HDV genotypes. We propose that comparative analysis of editing and HDAg-L function among the three HDV genotypes will provide valuable insight into the mechanisms by which HDV controls editing, and the relationship between editing levels and the role of HDAg-L in the HDV replication cycle. Because of the central role if editing in the HDV life cycle, this analysis is also likely to provide critical insights into the mechanistic basis for pathogenic variations among the HDV genotypes. The specific aims are: 1) to determine the contributions of different RNA structural elements to editing HDV RNA at the amber/W site in the three HDV genotypes; 2) determine the role and mechanism of RNA structural dynamics in HDV genotype III RNA editing and replication; 3) to determine the different mechanisms by which RNA editing is regulated in HDV genotypes I and III; and 4) to compare and contrast the functional properties of the viral protein HDAg-L in packaging and replication of HDV genotypes I and III. [unreadable] [unreadable] [unreadable]